SCOTT SIMON, HOST:
A miserable heat dome is over the South and Southwest this weekend. Temperatures creeping towards 120 degrees. Cities under the dome are especially hot with all those cars and asphalt and air conditioners. Brian Stone runs the Urban Climate Lab at Georgia Tech, and he's been studying how U.S. cities need to adapt as the climate warms. Mr. Stone, thanks so much for being with us.
BRIAN STONE: Great to be with you, Scott. Thanks so much.
SIMON: Humans, at least right now, can cool ourselves down from most temperatures, but is there a hot that's too hot?
STONE: There's definitely a hot that is too hot. It's a combination of both temperature and humidity. And so we measure this - what's being called wet-bulb temperature - at a combination of about 100 degrees and 60% relative humidity. That's the point where we lose the ability to cool ourselves by sweating, and that becomes, obviously, a very dangerous condition for us.
SIMON: Oh, mercy. What happens to cities in the 100-degree and 60% humidity range?
STONE: The good news for now is that it really doesn't happen often in the United States. We've looked in the Urban Climate Lab at Georgia Tech at large cities across the U.S. over about the last 20 years. Only a single city has hit a combination of temperature and humidity that crosses the threshold. It's not surprising that's Phoenix, actually. But there are many cities that are getting very close.
SIMON: When a city gets too hot, what happens?
STONE: Yeah. It's an extreme risk for U.S. cities. We can actually point to places in other parts of the world, like the Persian Gulf, where this critical threshold is regularly surpassed. I actually looked a moment ago, and Doha in Qatar today is exceeding this threshold. It would be unsafe to go outside in Doha at this moment. Even a young, healthy person would get a fever if they went outside for more than, say, an hour right now.
In these kinds of environments, you see the population has largely retreated inside for extensive portions of the year. And once we cross these thresholds, outdoor labor, garbage collection, athletic events, construction, all of this will have to cease. And so the threat is very substantial to us, and our timetable is not long.
SIMON: I have to ask, what cities may be especially at risk? You mentioned Phoenix.
STONE: Other large cities that are getting close, Chicago, Tampa, very close, Houston. There are six, seven, eight cities that really may cross this threshold in the next couple years.
SIMON: Atlanta, where you are?
STONE: Atlanta is fortunate to be at a relatively high elevation. We're at about 1,000 feet. We do have high humidity, but our temperatures rarely get to 100 degrees. And so we're not quite at that threshold, but like all cities, we are continuing to warm. And so, yeah, even in Atlanta, we will reach this threshold eventually.
SIMON: I understand that your city of Atlanta just passed a cool-roof ordinance. All new and replacement roofs have to be made of reflective and energy-efficient materials. What can this do?
STONE: Yeah. This is really exciting for Atlanta. We have had cool-roofing ordinances in many cities around the U.S., but typically they only focus on flat commercial, industrial roofs. So really, every roof in the city of Atlanta must meet this new standard, and that's going to both cool off the building and reduce the amount of energy that's needed to cool the building inside, but it will also cool off the air around the building.
SIMON: I know as a abiding Chicagoan, the trees are considered important, too, aren't they?
STONE: Yeah. Well, trees are critically important. Atlanta also is distinguished in the sense that we have a really extensive legacy canopy. And so Atlanta's got about a 45, 46% tree canopy, and that's pretty much the best in the country for a major city. But it is getting whittled down over time. As we develop, we are each year losing more trees than we're planting.
SIMON: Why are we losing more trees than we're planting?
STONE: Atlanta is a growing city, so we are losing trees to development. Much of our legacy canopy is quite old, and so, you know, trees get diseased and they get stressed by the increasing temperatures. And so we really have to be, I think, more creative and just, frankly, more aggressive in our programs to not just protect the trees we have, but to expand the tree canopy.
SIMON: But, of course, to point out the obvious, trees take decades, don't they?
STONE: They do take decades. It's fair to say that cool-roofing ordinance also takes decades and that we have to wait to replace all of these existing roofs. Both of these strategies are long-term strategies, and so we really need different kinds of policies and programs to complement a tree ordinance to be every year expanding the tree canopy beyond what we're losing each year.
SIMON: Are there any particular cities that are making progress when it comes to planting trees?
STONE: New York, you know, rather famously planted a million trees. They gave themselves 10 years to do it, and they did it in about seven to eight years. And today, you know, in the city of Atlanta, in our downtown district, we have about a 9% tree canopy cover. Manhattan, which is a much more dense environment, has a 21% tree canopy cover. And so it is possible to have extensive tree canopy, even in your most dense urban districts, but we have to understand tree canopy to be infrastructure. It requires real investment and a dedicated ongoing effort over time.
SIMON: Brian Stone runs the Urban Climate Lab at Georgia Tech in Atlanta. Thanks so much for being with us.
STONE: My total pleasure. Thank you. Transcript provided by NPR, Copyright NPR.
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